RE: [PATCH v3 12/35] x86/bugs: Restructure retbleed mitigation

Kaplan, David
2025-01-09T15:27:01Z

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
> Sent: Wednesday, January 8, 2025 11:23 PM
> To: Kaplan, David <David.Kaplan@amd.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>; Borislav Petkov <bp@alien8.de>; Peter
> Zijlstra <peterz@infradead.org>; Josh Poimboeuf <jpoimboe@kernel.org>; Ingo
> Molnar <mingo@redhat.com>; Dave Hansen <dave.hansen@linux.intel.com>;
> x86@kernel.org; H . Peter Anvin <hpa@zytor.com>; linux-kernel@vger.kernel.org
> Subject: Re: [PATCH v3 12/35] x86/bugs: Restructure retbleed mitigation
>
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>
> On Wed, Jan 08, 2025 at 02:24:52PM -0600, David Kaplan wrote:
> [...]
> > @@ -1064,6 +1063,7 @@ enum spectre_v2_mitigation spectre_v2_enabled
> > __ro_after_init = SPECTRE_V2_NONE;
> >
> > enum retbleed_mitigation {
> > RETBLEED_MITIGATION_NONE,
> > + RETBLEED_MITIGATION_AUTO,
>
> This new enum ...
>
> > RETBLEED_MITIGATION_UNRET,
> > RETBLEED_MITIGATION_IBPB,
> > RETBLEED_MITIGATION_IBRS,
> > @@ -1071,14 +1071,6 @@ enum retbleed_mitigation {
> > RETBLEED_MITIGATION_STUFF,
> > };
> >
> > -enum retbleed_mitigation_cmd {
> > - RETBLEED_CMD_OFF,
> > - RETBLEED_CMD_AUTO,
> > - RETBLEED_CMD_UNRET,
> > - RETBLEED_CMD_IBPB,
> > - RETBLEED_CMD_STUFF,
> > -};
> > -
> > static const char * const retbleed_strings[] = {
> > [RETBLEED_MITIGATION_NONE] = "Vulnerable",
> > [RETBLEED_MITIGATION_UNRET] = "Mitigation: untrained return thunk",
>
> ... does not have a corresponding entry in the strings array. AUTO is the default,
> and it is possible that mitigation mode can stay AUTO throughout the retbleed
> mitigation selection depending on cmdline and CONFIGs. e.g.
> retbleed=stuff and spectre_v2=off.

The intent was never to allow AUTO to persist, it should always be turned into a real mitigation. However it looks like I did miss a case there, where if the mitigation is AUTO when retbleed_select_mitigation() is called, the bug should be mitigated but the vendor isn't AMD/Hygon, it wasn't being transformed.

I'll figure out how to fix this to match the existing functionality, thanks for pointing this out.

--David Kaplan

>
> Other issue is below print in retbleed_update_mitigation() will dereference a NULL
> pointer:
>
> pr_info("%s\n", retbleed_strings[retbleed_mitigation]);